As a 9-year-old on an island between Hawaii and Australia, Tony deBrum witnessed the explosion of the largest bomb ever detonated by the United States. The “Castle Bravo” nuclear weapon was 1,000 times as powerful as the one dropped on Hiroshima in 1945.

From his perch 280 miles from ground zero, Mr. deBrum saw the flash of light — silent and brighter than the sun — and watched the sky turn red as blood. The terrifying thunder from the test explosion stayed with him the rest of his life, which he devoted to representing the people of the Republic of the Marshall Islands at home and abroad.

Mr. deBrum, who helped gain his nation’s independence from the United States — and then helped sue the United States for allegedly breaching an international treaty on nuclear nonproliferation — died Aug. 22 in Majuro, the capital city of his Pacific island nation. He was 72.

His death was announced by Hilda C. Heine, the president of the Marshall Islands, but the cause was not specified.

“He fought for our independence, he fought against the tyranny of nuclear weapons and for nuclear justice for our people, and he led the international fight against climate change,” Heine said in a statement. “The very existence of the Paris agreement owes a lot to Tony deBrum.”

Mr. deBrum’s diplomacy reached far beyond the remote Marshall Islands, a sprinkling of 29 coral atolls across 700,000 nautical square miles in the Pacific Ocean. There, the United States tested 67 high-yield nuclear bombs between 1946 and 1958, resettling whole islands of Marshallese people, exposing many to radioactive fallout and bequeathing exile and ill health to ensuing generations.

“I wonder how many in this room have actually witnessed a detonation of a nuclear weapon,” Mr. deBrum said to 191 nations in the U.N. General Assembly hall in April 2015, when he was the republic’s minister of foreign affairs. He paused for effect, then continued: “I have.” The Marshallese people “still carry a burden which no other people or nation should ever have to bear.”

Mr. deBrum was the public face of a 2014 lawsuit filed by the Marshall Islands against the United States and the eight other nuclear-armed countries, alleging noncompliance with the 1968 Nuclear Non-Proliferation Treaty.

“Our people have suffered the catastrophic and irreparable damage of these weapons, and we vow to fight so that no one else on Earth will ever again experience these atrocities,” Mr. deBrum said at the time, as the United States embarked on a $1 trillion refurbishment of its nuclear arsenal. Some saw the lawsuit as a courageous David vs. Goliath gambit, while others viewed it a frivolous stunt that damaged the republic’s relationship with the United States, which grants the country tens of millions of dollars every year in the form of environmental, food and health-care programs.

The lawsuit, which was dismissed, “is not done with acrimony or flippant disregard for the friendship” with the United States, Mr. deBrum said in an interview with The Washington Post in Majuro in March 2015. “The friendship is very important. That is probably why we do it.”

Tony Anton deBrum was born Feb. 26, 1945, in what is now the Pacific island nation of Tuvalu, and grew up in the Marshall Islands during the period of U.S. nuclear testing.

He was one of the first Marshallese people to attend college abroad, according to Heine. He studied history and linguistics at the University of Hawaii at Manoa, receiving a bachelor of arts in psychology in 1968.

He returned to the islands and entered public service in 1970 while the nation was still a territory of the United States. He served as an interpreter for U.S. representatives over the years and was first elected to the Marshallese parliament in 1984. He participated in the negotiation of the 1986 “compact of free association,” through which the United States granted the Marshall Islands independence and a $150 million settlement.

As foreign minister, Mr. deBrum traveled the world, armed with a moral authority rooted in his country’s nuclear past and its future tied to climate change.

A global temperature increase of “over 2 degrees Celsius is a death warrant for us,” Mr. deBrum told NPR in December 2015, during the Paris climate accord negotiations. “It means the islands go under.”

The average land elevation in the Marshall Islands is six feet above sea level. During Mr. deBrum’s tenure as foreign minister, the U.S. Embassy in Majuro considered the Marshall Islands to be on the front lines of climate change.

“He helped to bring together a large coalition of nations, the ‘High Ambition Coalition,’ that strengthened the [Paris] agreement and perhaps even made it possible,” said John Burroughs, executive director of the Lawyers Committee on Nuclear Policy, as he accepted an arms-control award for Mr. deBrum in Washington in June.

Also in June, President Trump announced that he would pull the United States from the climate accord.

In 2016, the International Peace Bureau nominated Mr. deBrum for the Nobel Peace Prize, and the Marshallese parliament drafted a resolution of support, citing his “great ambition” to raise awareness of “the devastating and destructive effects of climate change as well as nuclear testing.”

Survivors include his wife, Rosalie; three children; his father; 10 grandchildren; and five great-grandchildren.

For Mr. deBrum, climate change and the threat of nuclear weapons were twin existential crises, inextricably linked and embodied by his island nation.

“We’re suffering the result of climate change and of the nuclear legacy, and we have had nothing to do with either,” Mr. deBrum told The Post. “In either case, people have to choose to end this world, this universe. You can either do it slowly with climate change, or you can press a button and blow it up. And neither is justified.”